1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a copying apparatus or a facsimile apparatus, and more particularly to an image forming apparatus which executes a potential control on a surface of an image bearing member by a scorotron charger.
2. Related Background Art
As a charger for uniformly charging a surface of an image bearing member (such as an electrophotographic photosensitive drum) employed in an image forming apparatus such as a printer, a copying apparatus or a facsimile apparatus, a scorotron charger has been widely employed.
A scorotron charger charges an electrophotographic photosensitive member to a desired surface potential by means of a current or a voltage applied to a discharge wire for causing a discharge, and a current or a voltage applied to a grid for regulating the potential on the image bearing member at a predetermined potential. There have been proposed a method of fixing the voltage applied to the grid and regulating the current or the voltage applied to the discharge wire thereby obtaining a desired surface potential, and a method of fixing the current or the voltage applied to the discharge wire and regulating the current or the voltage applied to the grid thereby obtaining a desired surface potential.
For fixing the voltage applied to the grid, there is also widely employed a method of employing a constant voltage element such as a varistor in combination with a high voltage source and regulating the potential of the grid at a predetermined value through an exchange of charge from the discharge wire.
Also for regulating the current or the voltage of the discharge wire, there is available a device capable of regulation for example with a switch, and there is also proposed a circuit having a variable resistor combined with a varistor for changing a voltage applied to the grid.
However, the method of fixing the voltage applied to the grid is associated with following drawbacks. In case of regulation to a high surface potential, it is necessary to select an excessively high voltage or current to the discharge wire, since the grid voltage is fixed. As a result, there results a drawback that the discharge wire is conspicuously smeared. On the other hand, in case of regulation to a low surface potential in a state where the grid voltage is set high in order to avoid the aforementioned phenomenon, it is necessary to reduce the voltage or the current to the discharge wire, and, as a result, a smear on the discharge wire is reproduced on an image. Therefore, in case of fixing the voltage applied to the grid, it is not possible to obtain a sufficiently wide regulating range of the surface potential.
On the other hand, in case of fixing the current or the voltage of the discharge wire and executing a regulation by the current or the voltage of the grid, a high current or voltage of the grid results in a large potential difference between the grid and the photosensitive member, thereby tending to induce a spark discharge therebetween. As a result, a high voltage or a high current cannot be applied so that a high surface potential cannot be obtained on the photosensitive drum. On the other hand, it is also conceivable to increase a current or a voltage to the discharge wire, but a constantly increased current to the discharge wire is undesirable as it aggravates the smear on the discharge wire. Therefore, in case of fixing the current or the voltage applied to the discharge wire, as in the method of fixing the voltage to the grid, it is not possible to widen the regulating range of the surface potential.
On the other hand, Japanese Patent Application Laid-open No. 06-274012 discloses a method in which a main charging voltage of the discharge wire and a grid voltage are both made variable in order to widen the regulating range of the surface potential. In this method, plural characteristics of the main charging voltage and the surface potential of the photosensitive member at different grid voltages are activated in succession, and a desired surface potential is realized at a main charging voltage as low as possible in such characteristics.
This method, however, has a drawback of requiring a long control time before an appropriate state can be obtained, as plural characteristics are activated in succession. Also a low voltage applied to the discharge wire as a main charging voltage requires a constantly higher grid voltage, thereby depriving the grid of its inherent rectifying effect. For this reason, in case a discharge unevenness is caused by a deterioration in time for example in the discharge wire, the rectifying effect of the grid is suppressed to result in a drawback such as an unevenness in the potential of the photosensitive member.